1. Field of the Invention
The present invention relates to a thin film aluminum alloy used as a conductive thin film material for flat type display circuits, such as a liquid crystal display and for semiconductor integrated circuits, and to a sputtering target to form the thin film aluminum alloy.
2. Description of the Related Art
Either aluminum type alloys or copper type alloys are currently used as thin film electrodes and wire materials for semiconductor elements and liquid crystal displays. Among them, copper type alloys have insufficient adhesion to an oxide film and unsatisfactory corrosion resistance and involve difficulties in plasma etching and are therefore used solely for devices in specific use. Aluminum alloys are therefore usually used. Such aluminum materials are required to be decreased in specific resistance and to have hillock resistance. The request for the decrease in specific resistance among these characteristics is intended to prevent signal delay along with accelerated trends to up-sizing and high refinement of wiring width in the fields of recent liquid crystal displays as well as semiconductor elements. On the other hand, the hillock resistance is needed to prevent fine projections (hillocks) from being formed on the film surface due to internal stress caused by Al which inherently has low heat resistance in heating treatment (annealing treatment) after forming electrodes and wiring material films when using an aluminum alloy as electrode and wiring materials having a low specific resistance.
As aluminum type alloys having this type of hillock resistance, aluminum alloys which are made to contain, as an alloy component, based on Al at least one type selected from Nd, Gd and Dy in an amount exceeding 1.0 atom % and up to 15 atom %, are known as shown in Japanese Patent No. 2733006. Eventually, aluminum type electrode materials formed on a substrate for a liquid crystal display suffer from the occurrence of hillocks through heating treatment (annealing treatment) which is inevitable in the production step of the device. For these electrode materials, aluminum alloys are formed by adding Nd and the like to Al for the purpose of improving heat resistance due to a solid solution effect.
Also, for example, Japanese Patent Application Laid-Open Specification No. 2000-235961 discloses a conductive thin film formed using an aluminum alloy comprising Al as its major component, at least one element selected from Zr, Hf, Cu, Ti, Mo, W, Fe, Cr and Mn in a content of 0.5 to 1.5 atom % as a metallic element being sub-component and at least one element selected from Si and Ge in a content of 0.5 to 1.5 atom % as a semiconductor element forming an alloy in combination with Al. This alloy is made to contain a metallic element for the purpose of preventing the occurrence of hillocks by segregating the metallic element at the grain boundary and to contain a semiconductor element for the purpose of preventing the occurrence of hillocks by allowing the element to form an alloy in combination with Al.
All of the foregoing current aluminum alloys improve heat resistance by reinforcement due to a solid solution effect and segregation effect obtained by adding other elements to Al. In this case, the raw materials of the aluminum alloys are anneal-treated, because the above solid solution effect and segregation effect bring about an increase in specific resistance at the same time. This process ensures that the elements existing as solid solution in Al precipitate, so that the total amount of the solid solution of the elements put in a solid solution, which is a cause of an increase in specific resistance, decreases and the specific resistance therefore falls.
However, the reduction in specific resistance caused by annealing treatment as mentioned above depends upon the temperature under annealing treatment. Therefore, there is a possibility that no aluminum alloy having a desired low resistance is obtained in relatively low temperature (about 350xc2x0 C. or less) among annealing treatment conditions. Further, it is difficult to converge the specific resistance to a specified range irrespective of ambient temperature conditions, because the specific resistance varies depending on the heat treatment temperature.
Also, if the content of elements to be added to obtain solid solution and segregation effects is high, the hardness of the film formed using an electrode film-forming sputtering target comprising such an aluminum alloy tends to be high. Such an increase in hardness can be suppressed by the precipitation of the added elements through annealing treatment. However, the hardness varies depending upon the temperature under annealing treatment in the same manner as in the case of the above specific resistance and it is therefore difficult to converge the hardness to a specified range.
In view of the above problems, it is an object of the present invention to provide a thin film aluminum alloy which is limited in the occurrence of hillocks, in which stable low specific resistance is consistently maintained irrespective of annealing treatment temperature, and also to provide a sputtering target used to form the thin film aluminum alloy.
In order to solve the above problems, a thin film aluminum alloy according to the present invention is designed to have a Vickers hardness of 30 Hv or less and a film stress (absolute value indication) of 30 kg/mm2 or less when performing annealing treatment at 25xc2x0 C. to 500xc2x0 C. and such physical properties that the above hardness and film stress are distributed in a predetermined hardness range and in a predetermined film stress range respectively within the temperature range of the above-mentioned annealing treatment. The aluminum alloy has the characteristics that the above-mentioned hardness and film stress which inherently vary depending on annealing temperature are distributed in a small variation range within the temperature range of the above-mentioned annealing treatment, so that the dependencies of the hardness and stress on annealing temperature may be neglected approximately and both may be therefore almost regarded as constant against the annealing temperature. In addition, the occurrence of hillocks can be restrained and a low specific resistance, which Al inherently has, can be maintained, because a thin film having a small residual stress (film stress) and a low hardness is formed.
Incidentally, the film stress is expressed as compressive stress when it is minus and as tensile stress when it is plus.
In this case, the thin film aluminum alloy of the present invention preferably comprises, as alloy components, 0.5 to 15 atom % of one or more types selected from Ag, Cu, Mg and Zn, 0.01 to 5 atom % of one or more types selected from Co, Cr, Gd, Hf, Li, Mn, Mo, Nb, Nd, Ni, Pd, Pt, Ru, Sc, Sr, Ta, Ti, W, Y and Zr, and, as remnant, Al and unavoidable impurities. An elemental group consisting of Ag, Cu, Mg and Zn is added as a crystalline nucleus in the above alloy to increase the density of nucleic generation. On the other hand, another elemental group consisting of Co, Cr, Gd, Hf, Li, Mn, Mo, Nb, Nd, Ni, Pd, Pt, Ru, Sc, Sr, Ta, Ti, W, Y and Zr segregates at the grain boundary of the above alloy to prevent crystalline particles from being coarsened. The addition of each of the above elements brings about the result that a fine structure in the aluminum alloy is maintained under annealing and the residual stress is limited to such a small level as mentioned above that the generation of hillocks is restrained, in which a low hardness and a low specific resistance are maintained.
When the content (composition ratio) of at least one or more elements among an elemental group consisting of Ag, Cu, Mg and Zn is less than 0.5 atom % or the content (composition ratio) of at least one or more elements among an elemental group consisting of Co, Cr, Gd, Hf, Li, Mn, Mo, Nb, Nd, Ni, Pd, Pt, Ru, Sc, Sr, Ta, Ti, W, Y and Zr is less than 0.01 atom %, too many hillods are generated to be practical. On the other hand, when the content (composition ratio) of at least one or more elements among an elemental group consisting of Ag, Cu, Mg and Zn exceeds 15 atom % or the content (composition ratio) of at least one or more elements among an elemental group consisting of Co, Cr, Gd, Hf, Li, Mn, Mo, Nb, Nd, Ni, Pd, Pt, Ru, Sc, Sr, Ta, Ti, W, Y and Zr exceeds 5 atom %, the problems arise in that the hardness and specific resistance become higher.
Further, the thin film aluminum alloy ensures that Al and unavoidable impurities including the above-mentioned alloy components can be formed on the substrate by a sputtering method. It is noted that generally the thin film formed by sputtering is used after anneal treatment. But the thin film aluminum alloy of the present invention does not consistently require annealing treatment after sputtering as mentioned above, because it has almost no dependency on annealing temperature, whereby the thin-film forming step can be simplified. (The range of temperature at which an annealing effect is obtained is shifted to a lower temperature side and a similar annealing effect can be expected either at lower temperatures, for example, room temperature (about 25xc2x0 C.) or at higher temperatures.)
In the meantime, it is assumed that a thin film aluminum alloy having hillock resistance as mentioned above is supposed to develop superplastic deformation, eventually, a phenomenon that grain boundaries formed by fine crystals adjacent to each other in the alloy generate numerous grain boundary slips under the deformation to loosen stress concentration inside of the alloy. This implies that the thin film aluminum alloy has superplastic deformation characteristics. The thin film aluminum alloy of the present invention has a film thickness of several microns or less and is formed on a substrate as the case may be. It is therefore impossible to directly measure the superplatic deformation which is usually measured as huge elongation of a material. Therefore, the hardness characteristics and film stress (internal residual stress) characteristics were measured as the superplastic deformation characteristics to grasp indirectly that the thin film aluminum alloy had the superplastic deformation characteristics. It is noted that the foregoing annealing effect obtained at room temperature is also caused by the superplastic deformation characteristics.
Also, it is suitable to use as electrode or wiring materials in semiconductor elements or liquid crystal displays, because the thin film aluminum alloy obtained in this manner can restrict the number of hillocks with a low specific resistance.
Moreover, a sputtering target to form such a thin film aluminum alloy by a sputtering method can be constituted of an aluminum alloy comprising 0.5 to 15 atom % of at least one element selected from an elemental group (Ag, Cu, Mg and Zn) used to increase the density of the generation of crystalline nucleus of the aluminum alloy and 0.01 to 5 atom % of at least one element selected from an elemental group (Co, Cr, Gd, Hf, Li, Mn, Mo, Nb, Nd, Ni, Pd, Pt, Ru, Sc, Sr, Ta, Ti, W, Y and Zr) used to prevent crystalline grains of the alloy from being coarsened, and, as remnant, Al and unavoidable impurities. A thin film formed by sputtering with this target remains a low specific resistance, shows a low hardness and is restrained in the occurrence of hillocks after anneal treatment.
It is noted that the composition of the target does not consistently coincide with that of the thin film obtained by sputtering the target, because of other sputtering conditions.
Also, in the sputtering method, a crystal grain growth by the elements (one or more types among Ag, Cu, Mg and Zn) to promote the generation of crystal nucleus, which is secured to the substrate after flying, proceeds in parallel to a restriction on the growth of grain diameter by the elements (one or more types among Co, Cr, Gd, Hf, Li, Mn, Mo, Nb, Nd, Ni, Pd, Pt, Ru, Sc, Sr, Ta, Ti, W, Y and Zr) to prevent the crystalline grains from being coarsened, which is secured to the substrate after flying likewise from the target. As a result, the crystal maintains a fine structure in which crystals are adjacent to each other through numerous grain boundaries. Eventually, the circumstance under which the thin film is formed by a sputtering method is the same as that under which superplastic deformation based on the generation mechanism caused by numerous grain boundary slips develops. The adoption of a sputtering method brings a major factor to provide the aluminum alloy of the present invention with superplastic deformation characteristics.